(1) Field of the Invention
The present invention relates to a liquid-filled type vibration damping structure for effectively damping or isolating a vibration by the damping effect of liquid, and more particularly to a liquid-filled type vibration damping structure in which a residual frequency peak of a vibration transmission characteristic caused by a movable plate can be cancelled by a bottoming frequency portion of an another vibration transmission characteristic of a liquid-filled vibration isolator.
(2.) Description of the Prior Art
A liquid-filled type vibration isolator is, in general, constituted by dividing into two liquid chambers the internal space of a resilient member provided between a pair of frames by means of a baffle member attached to one of the frames and by defining an orifice through the baffle for communication between the two liquid chambers. Such a vibration isolator is provided between a power unit and a vehicle body in mounting the former on the latter.
The conventional liquid-filled type vibration isolator is designed to damp a high-frequency vibration through the vibration of the baffle member with a movable plate provided, as disclosed in Japanese Patent Disclosure No. 5376/1978. That is the orifice defined through the baffle provides the effect of a damper to damp a vibration between two liquid chambers upon a low-frequency band vibration being transmitted from the power unit.
On the other hand, a high-frequency band vibration from the power unit is damped by the vibration of movable plate permitting an extremely small voluminal change between two liquid chambers and by the vibration absorbing effect of the resilient member.
In such a liquid-filled type vibration isolator, however, a portion of the liquid present in the moving range of the movable plate is caused to form a sort of mass body for generating a liquid column resonance upon the movable plate vibrated as a result of high-frequency band vibration, and thereby producing a vibration transmission characteristic A shown in FIG. 1. That is, while such a vibration transmission characteristic A shows an effective vibration damping result around the frequency of 100 Hz, it causes a substantially large frequency peak P to appear around the frequency of 200 Hz, thereby generating a booming sound confined sound inside a vehicle.
To solve such a problem, it may be proposed that the resilient member of a liquid-filled type vibration isolator is provided with a mass body to utilize a vibration transmitted under the mass damping effect of mass body caused by the spring action of resilient member in cancelling the frequency peak P. In this proposal, however, the need for the resilient member to support the static load of power unit itself prevents the resilient member from employing a small spring constant.
To this end, the mass body must have a substantially enlarged mass to provide a sufficient mass damping effect. This naturally results in a vibration isolator having an increased dimensional and weight requirement such that it would be impractical for actual application.